Organic pigments consist of very fine particles, of low solubility in customary solvents, whose dimensions can lie within the range from submicroscopic to about 100 .mu.m. For practical use, organic pigments having approximate particle sizes of from 0.01 to 0.1 .mu.m for transparent forms and from 0.1 to 10 .mu.m for opacifying forms have proved most suitable.
The physical properties of the pigment particles are very important to their use. For instance, very small particles possess an often relatively low light fastness and fastness to weathering and a strong propensity to agglomeration. Very coarse particles, on the other hand, give rise to undesirably low colour intensities and duller shades. In the case of the physical properties, however, particle size distribution and agglomeration play a key part, especially with respect to the dispersibility of the pigments [cf. Farbe und Lack 82/1, 7-14 (1976)].
It is therefore of critical importance for pigments to have a very narrow particle size distribution, which can usually be achieved by reprecipitation, recrystallization or heat treatment in a polar solvent, at atmospheric or superatmospheric pressure or under a high shear force (U.S. Pat. No. 4,879,380). However, all such pigments, irrespective of the narrow particle size distribution, still have the great disadvantage of producing dust. Consequently, when they are used, expensive measures (for example of a workplace safety, ecological or quality assurance nature) are necessary and valuable material is lost.
A very large number of methods have therefore already been investigated for converting the pigments into a low-dust or even dust-free form. It has however been found that improvements in the dusting behaviour can be obtained in the case of the known methods only, among other disadvantages, at the expense of the physical properties of the pigment particles, and especially at the expense of the dispersibility. Consequently, the known methods described below are unable to satisfy fully the long-held wish for dust-free organic pigments which continue to have good physical properties.
Known compacting methods, such as compression moulding (tableting or briquetting), granulation in mix granulators and granulating discs (Aufbereitungs-Technik 12 (1975)) and in formers (Chem.-Ing.-Tech. 49/5, 374-380 (1977)), roll granulation (DE-A 27 23 221) or pressure granulation (Powder Technology 74, 1-6 (1993)) always lead, with pure organic pigments, to highly agglomerated products having performance properties worse than those of the powder. A common feature of these methods is that the pigment particles collide with one another with a relatively high force.
Pigments for use in plastics can be incorporated into polymer concentrates. The pigment is employed as a dry, dusting powder. In this context, high shear forces and temperatures are required to disperse the pigment particles thoroughly, and the physical properties and colour properties are changed. The resulting polymer grains must in turn be mixed intimately with uncoloured polymer grains for the end use, again under high shear force, since it is necessary to effect homogeneous distribution of the pigment particles together with the completely surrounding polymer. Moreover, the concentrate polymer must be compatible with the other polymer, which is why, for a single pigment, a range of two or more products is required for different plastics applications.
Pigments can also be applied to the surface of externally softened polymer granules to give spherical particles (U.S. Pat. No. 4,310,483). However, the size of such particles is difficult to control, and the fraction having the desired diameter has to be isolated by sieving. It is said that the granulating auxiliary can be used in amounts of 2-50% by weight (preferably 5-30% by weight), although it has been found that good dispersibility can only be achieved with amounts of at least 15-20% by weight. An additive which can be used in addition to the polymer granules is a wax-like binder whose melting point is typically from 49 to 88.degree. C. (U.S. Pat. No. 5,455,288). In the latter case, however, the pigment content is at an unsatisfactorily low level of from 5 to 50%. In both cases, collision forces are principally at work in the case of low shear forces, and the presence of more than 10% by weight of a substance of low melting point is disadvantageous from the performance standpoint.
Pigments can also be embedded in resins. This is done by first preparing a dispersion of the pigment in an inert solvent (for example water) and a solution of the resin in an appropriate solvent and then mixing the two, and precipitating the resin from the solution, either directly in the course of mixing or else later, the pigment being enveloped by the precipitating resin. innumerable publications have proposed, as the resin, almost all substances known to have a certain resinous character, including rosin. Various processes in accordance with this principle are known, for example acid/base precipitations (CS 216 590; IN 156 867; DE-A 33 27 562) and one- or two-phase solvent granulations (U.S. Pat. No. 4,055,439; U.S. Pat. No. 4,208,370). The not entirely satisfactory dispersibility of such resin-embedded pigments can be improved by the use of special resin mixtures coupled with a very high shear force (U.S. Pat. No. 4,116,924; U.S. Pat. No. 4,168,180). Nevertheless, the preparation of the pigment dispersion in any case requires intensive milling, especially when the inert solvent used is an aqueous medium, in which case the pigment, however, is comminuted in an undesirable manner. Instead of resins it is also possible to use surfactants (EP 403 917): in this case, although dispersion is made easier, the product is not obtained in a dust-free form but rather in a powder form.
In the case of the acid/base precipitations, the control of neutralization is a further problem which cannot be solved with complete satisfaction by the method described in DE 33 27 562. When precipitating with acid, in fact, the resin does not precipitate in a completely neutral form, which in many cases causes problems for high-grade applications, such as coating operations or the mass colouring of plastics. In the case of solvent granulations, on the other hand, large amounts of solvents are required which, disadvantageously, have to be recovered from usually aqueous mixtures. Therefore, the use of acetic or propionic anhydride as solvent has been proposed (EP 069 617), giving rise to aqueous solutions which can purportedly be used in the chemical industry but which, for lack of demand, have to be disposed of at considerable cost.
It is also known that colorants can be converted into a low-dust flowable form by spray-drying or in a fluidized bed (EP 039 841; EP 670 352). The additives used therein, however, are completely unusable in the case of pigments that are to be used in high-grade applications such as the mass colouring of plastics or automotive finishing operations. Moreover, in the case of spray-drying or in a fluidized bed it is hardly possible to prepare homogeneous granules having a particle size of more than a few 100 .mu.m (cf. e.g. Chemie-Technik 2116, 72-78 (1992); Arch. Pharm. Chemi, Sci. Ed. 1978/6, 189-201). Furthermore, it is not possible to fluidize all powders in a fluidized bed (Powder Technology 57, 127-133 (1989)), so that it is in no way possible to make generalized use of this method.
An improved variant of fluidized bed granulation, especially for pigments, is also known (U.S. Pat. No. 4,264,552) where the particle size distribution of these granules is very broad and the great majority of the particles (about half by weight) are smaller than 500 .mu.m. Furthermore, these granules still have an excessive propensity to produce dust. In Example 2 the use is disclosed of a mixture of 8.2% by weight of Staybelite Resin.TM. and 0.9% by weight of hydroxypropylcellulose (amounts based in each case on the finished product), in the form of its ammonium salts, as the anionic surfactant.
Water-soluble dyes can be processed with from 5 to 50% by weight of a water-soluble binder to form non-dusting cylindrical granules with a diameter of at least 1 mm (DE-A 2 317 175); according to the examples the granules have a diameter of about 1 mm and a length of 5-7 mm. By means of a conveying screw, the homogeneous plastic mass is pressed through a perforated disc (perforation diameter 1 mm).
However, water-soluble binders are completely unsuitable for pigments that are intended for use in customary plastics, and organic pigments treated by this process are highly agglomerated and have unsatisfactory dispersibility properties. They are therefore not sufficiently suitable for many applications. The same applies to the powder compression of water-moist pigments in a twin-screw extruder at pressures from 10 to 50 bar in accordance with the method known from Journal of Powder & Bulk Solids Technology 4/4, 27-32 (1980).
Also known are fine, low-dust colorant granules having a very low content of wax-like binder and other foreign substances (EP 424 896). Of essential importance is the use of a device in which the material fed in is exposed predominantly to severe turbulence and moderate collision forces coupled with declining shear stresses. This process is suitable, however, predominantly for inorganic pigments, and only one example of an organic pigment is disclosed: Example 13 uses the monoazo pigment Pigment Red 176, 0.72% by weight of a fatty acid mixture having a melting point of 57-61.degree. C. as the wax, and 50-51% by weight of water (based in each case on the fine granules). The particle size is markedly less than 1 mm, and a sieving operation is necessary despite the relatively small proportions of coarse particles, which are difficult to disperse, and ultrafine particles, which produce dust.
Other known granules include those which exhibit a large increase in volume relative to the initial pigment as a result of the cavities which are retained on drying (EP 510 392). In this case shaping takes place exclusively by known methods; for extrusion to strands a residual moisture content of 50 to 80% by weight is specified. These granules are said to be readily dispersible and low in dust, but are brittle and have a low bulk density, with the result that they take up a substantially greater volume on transportation and storage. Furthermore, it is very difficult to obtain products having a precisely reproducible specific weight, and in the case of hydrophobic or apolar pigments this method produces unsatisfactory results.
Also known, finally, is a process in which hydrophilic pigments are transported as aqueous, paste-like agglomerates (U.S. Pat. No. 5,328,506). In contrast to extruded "noodles" it is not necessary to carry out high-energy dispersion of these products later on. However, the process is aimed not at organic pigments but at inorganic kaolin pigments, and the presence of water in the stated amount of 1 to 25% has an adverse effect on, or may even rule out totally, the use of organic pigments in the vast majority of fields.
Furthermore, as already mentioned, the coarsely particulate pigment preparations prepared by some of the known processes are still not, simultaneously, satisfactorily compact, dust-free and/or readily dispersibile. In addition, the desired particle size usually has to be selected by sieving, with particles having a size other than this desired particle size (especially the fine fraction) having to be passed back to the process. However, the recycling of the unsatisfactory pigment material causes an additional detrimental alteration to its physical parameters, and, consequently, a worsening in the performance properties as well.
The aim of the invention was to provide coarsely particulate, extremely dust-free, highly concentrated, readily dispersible and universally applicable organic pigment granules in which, apart from the external aspect, the physical parameters of the pigment particles are changed as little as possible relative to the initial pigment powder, unlike the known granules. The term physical parameters is understood as meaning not only the abovementioned properties but also all other technically measurable or applications-relevant properties. The intention is that these pigment granules should as far as possible be able to be prepared by means of simple and universally applicable methods, in simple, inexpensive and easy-to-clean apparatus, without the addition of organic solvents to be absolutely necessary and without the need to select the right particles and recycle the rejects.